▪ Abstract Plant responses to salinity stress are reviewed with emphasis on molecular mechanisms of signal transduction and on the physiological consequences of altered gene expression that affect biochemical reactions downstream of stress sensing. We make extensive use of comparisons with model organisms, halophytic plants, and yeast, which provide a paradigm for many responses to salinity exhibited by stress-sensitive plants. Among biochemical responses, we emphasize osmolyte biosynthesis and function, water flux control, and membrane transport of ions for maintenance and re-establishment of homeostasis. The advances in understanding the effectiveness of stress responses, and distinctions between pathology and adaptive advantage, are increasingly based on transgenic plant and mutant analyses, in particular the analysis of Arabidopsis mutants defective in elements of stress signal transduction pathways. We summarize evidence for plant stress signaling systems, some of which have components analogous to t...

Plant cellular and molecular responses to high salinity.

Salt tolerance and salinity effects on plants: a review.

Cold, salinity and drought stresses: an overview.

The human ATP-binding cassette (ABC) transporter superfamily

Environmental stresses come in many forms, yet the most prevalent stresses have in common their effect on plant water status. The availability of water for its biological roles as solvent and transport medium, as electron donor in the Hill reaction, and as evaporative coolant is often impaired by environmental conditions. Although plant species vary in their sensitivity and response to the decrease in water potential caused by drought, low temperature, or high salinity, it may be assumed that all plants have encoded capability for stress perception, signaling, and response. First, most cultivated species have wild relatives that exhibit excellent tolerance to abiotic stresses. Second, biochemical studies have revealed similarities in processes induced by stress that lead to accumulated metabolites in vascular and nonvascular plants, algae, fungi, and bacteria (Csonka, 1989; Galinski, 1993; Potts, 1994). These metabolites include nitrogen-containing compounds (proline, other amino acids, quaternary amino compounds, and polyamines) and hydroxyl compounds (sucrose, polyols, and oligosaccharides) (McCue and Hanson, 1990). Accumulation of any single metabolite is not restricted to taxonomic groupings, indicating that these are evolutionarily old traits. Third, molecular studies have revealed that a wide variety of species express a common set of genes and similar proteins (for example, Rab-related proteins and dehydrins) when stressed (Skriver and Mundy, 1990; Vilardell et al., 1994). Although functions for many of these genes have not yet been unequivocally assigned, it is likely, based on their characteristics, that these proteins play active roles in the response to stress. Learning about the biochemical and molecular mechanisms by which plants tolerate environmentat stresses is necessary for genetic engineering approaches to improving crop performance under stress. By investigating plants under stress, we can learn about the plasticity of metabolic pathways and the limits to their functioning. Also, questions of an ecological and evolutionary nature need investigation. Are the genes that confer salt tolerance on halophytes and/or drought tolerance on xerophytes evolutionarily ancient genes that have been selected against in saltand drought-sensitive plants (glycophytes) for the sake of productivity? Or have some species obtained nove1 genes in their evolutionary history that have enabled them to occupy stressful environments? How will the

Adaptations to Environmental Stresses.

Salt stress is primarily osmotic stress, and halophilic/halotolerant microorganisms have evolved two basic mechanisms of osmoadaplation: the KCI-type and the compatible-solute type, the latter representing a very flexible mode of adaptation making use of distinct stabilizing properties of compatible solutes. A comprehensive survey, using HPLC and NMR methods, has revealed the full diversity of euhacterial compatible solutes found in nature. With the exception of proline (a proteinogenic amino acid) they are characterized as amino acid derivatives of the following types: betaines, ectoines, N-acetylated diamino acids and N-derivatized carboxamides of glutamine. From our present knowledge of hiosynthetic pathways it appears that, apart from glycine betaine, all nitrogen-containing compatible solutes originate from two major pathways (the aspartate branch and the glutamate branch). Uptake of compatible solutes from the growth medium (environment) seems to have preference over de novo synthesis. Therefore in the natural ecosystem the solutes of primary producers (mainly glycine betaine), which are readily excreted upon dilution stress, certainly play an important role as a ‘preferred’ solute source for heterolrophic organisms, and as a ‘vital’ source for organisms unable to synthesize their own compatible solutes.

Microbial behaviour in salt‐stressed ecosystems

The accumulation of compatible solutes is a prerequisite for the adaptation of microorganisms to osmotic stress imposed by salt or organic solutes Two types of strategies exist to cope with high external solute concentrations; one strategy is found in the extremely halophilic Archaea of the family Halobacteriaceae and the Bacteria of the order Haloanaerobiales involving the accumulation of inorganic ions The other strategy of osmoadaptation involves the accumulation of specific organic solutes and is found in the vast majority of microorganisms The organic osmolytes range from sugars, polyols, amino acids and their respective derivatives, ectoines and betaines The diversity of these organic solutes has increased in the past few years as more organisms, especially thermophilic and hyperthermophilic Bacteria and Archaea, have been examined The term compatible solute can also be applied to solutes that protect macromolecules and cells against stresses such as high temperature, desiccation and freezing The mechanisms by whihh compatible solutes protect enzymes, cell components and cells are still a long way from being thoroughly elucidated, but there is a growing interest in the utilization of these solutes to protect macromolecules and cells from heating, freezing and desiccation

An overview of the role and diversity of compatible solutes in Bacteria and Archaea

The aim of this study was to elucidate the protective effect of the new compatible solutes, ectoine and hydroxyectoine, on two sensitive enzymes (lactic dehydrogenase, phosphofructokinase). The solutes tested also included (for reasons of comparison) other compatible solutes such as glycine betaine and a number of disaccharides (sucrose, trehalose, maltose). All compatible solutes under investigation displayed remarkable stabilizing capabilities. However, the degree of protection depended on both the type of solute chosen and the enzyme used as a test system. The most prominent protectants were trehalose, ectoine and hydroxyectoine, which are very often found in nature (singly or in combinationn) as part of the compatible solute “cocktail” of moderately halophilic eubacteria.

Enzyme stabilization be ectoine-type compatible solutes: protection against heating, freezing and drying

A novel cyclic amino acid was detected in and subsequently isolated from extremely halophilic species of the bacterial genus Ectothiorhodospira. The structure of this new compound was elucidated by a combination of nuclear magnetic resonance (NMR) techniques and mass spectrometry. 1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid is only accumulated within the cytoplasm under certain growth conditions and seems to serve an osmoregulatory function. There is no previous reference to this molecule in the chemical literature and we, therefore, propose to use the trivial name 'ectoine', due to its discovery in members of the bacterial genus Ectothiorhodospira. (formula: see text).

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1985.tb08903.x

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid. A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira.

A novel biotechnological process called "bacterial milking" has been established for the production of compatible solutes using the Gram-negative bacterium Halomonas elongata. Following a high-cell-density fermentation which provided biomass up to 48 g cell dry weight per liter, we applied alternating osmotic shocks in combination with crossflow filtration techniques to harvest the compatible solutes ectoine and hydroxyectoine. H. elongata, like other halophilic or halotolerant microorganisms, produces compatible solutes in response to the salinity of the medium. When transferred to a low salinity medium (osmotic downshock), H. elongata cells rapidly released their solutes to achieve osmotic equilibrium. Subsequent reincubation in a medium of higher salt concentration resulted in resynthesis of these compatible solutes and-after a defined regeneration time-the procedure could be repeated. By repeatedly performing this "bacterial milking" process (at least nine times) we were able to produce large amounts of ectoines with a biomass productivity of 155 mg of ectoine per cycle per gram cell dry weight. Further purification of the products was achieved by a simple two-step procedure based on cation exchange chromatography and crystallization. The principles described in this article may also be useful for the production of other low-molecular-weight compounds.

Erwin A. Galinski

Papers

Microbial behaviour in salt‐stressed ecosystems

An overview of the role and diversity of compatible solutes in Bacteria and Archaea

Enzyme stabilization be ectoine-type compatible solutes: protection against heating, freezing and drying

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid. A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira.

Bacterial milking: A novel bioprocess for production of compatible solutes.